doc/netlink/specs: Add draft nftables spec

Add a spec for nftables that has nearly complete coverage of the ops,
but limited coverage of rule types and subexpressions.

Signed-off-by: Donald Hunter <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: donaldh